The presence of substances harmful to the environment or health in drinking water, even in very small amounts (so called trace species), has become a focus of attention for individuals as well as authorities, as knowledge and analytical techniques improve. Arsenic (As) in drinking water, for example, increases the cancer risk of human beings. Therefore, authorities have lowered the limit for As in drinking water from 50 μg/l to 5 μg/l. However, this resulted in a large number of waterworks failing to comply with a limit of 5 μg/l using existing methods, and hence had either to close down or to invest in expensive purification equipment. To this day, no profitable methods for this purpose are known capable of reducing the content of arsenic from a frequently encountered level of 20-35 μpg/l down to below 5 μg/l. The problem is particularly pronounced in waterworks having groundwater with a low iron content.
In waterworks with groundwater having a high content of iron compounds, the problem is less pronounced, since arsenic is co-precipitated with oxidized iron compounds, when the water is treated in a conventional way by oxidation, typically aeration, until iron precipitates in sand filters or precipitation basins. However, it is not possible to remove arsenic by conventional oxidation of water, if the iron content of the water is not sufficient to ensure the desired co-precipitation of present arsenic and other contaminants, including pesticides.
DE 197 45 664 A1 discloses a method for treating arsenic-containing water, where the water flows through a reactor filled with an iron-containing granulate, said granulate being produced by mixing sand and iron powder and subsequent firing under exclusion of oxygen. In the reactor, the iron is oxidized by the oxygen dissolved in water generating Fe(III) ions, said ions together with As forming poorly soluble iron arsenate. Excess Fe(III) ions are precipitated as iron hydroxide binding As by adsorption. Thus, As binds to the granulate, wherefrom it has to be removed at suitable intervals. When precipitating Fe(III) compounds, the granulate used agglomerates comparatively quickly and has to be exchanged frequently. The manufacture of the granulate requires work and energy. Moreover, the method requires the supply of additional oxygen to the reactor prior to treatment, if the treated groundwater is low in oxygen. In conclusion; the known method is work-intensive, complicated and expensive.
U.S. Pat. No. 5,951,869 describes a reactor, where water is treated with iron while simultaneously supplying oxygen. The treatment takes place in a fluid bed with iron particles as the source of iron. The use of a fluid bed is a high-cost and complicated method.
EP 0 737 650 A2 and U.S. Pat. No. 5,368,703 provide the enrichment of water with iron using an electrolytic method. The use of electrolysis is energy-intensive and complicated. WO 98/57893 (Nikolaidis) discloses the treatment of arsenic containing water with metallic iron under anaerobic conditions. The arsenic compounds are reduced and the iron is oxidized providing iron ions leading to co-precipitation of arsenic compounds but still under anaerobic conditions.
U.S. Pat. No. 5,358,643 (McClintock) discloses treatment of arsenic contaminated water with an iron salt, an acid and an oxidant until the water contains more iron than arsenic and has a positive oxygen/reduction potential (ORP) of about plus 600 mV ensuring that the arsenic will be pentavalent (step a). Then the water is made basic (step b) and the formed reaction mixture is reacted to precipitate compounds of As and Fe (step c) followed by separation of the precipitates from the water (step d). The steps a and b and in a preferred embodiment also step c are carried out without exposing the water/reaction mixture to air. Thus, an intentional active oxidation during the co-precipitation of As and Fe is not disclosed.
US 2002/0003116 A1 (Golden) discloses treatment of arsenic containing water first with an oxidant (hydrogen peroxide) and thereafter with an iron salt (ferric sulfate).
JP 2003 126874 A (Hitachi) teaches to feed the water to be treated to an oxidizing vessel where the water is aerated with an aerating means.
The above-mentioned methods have the common feature that the iron treatment takes place concomitant with aeration or requires that the water has a suitable oxygen content from the very start. Accordingly, there is an increased risk that the system is clogged by the precipitated oxidized iron compounds.
Hence, there is still a need for an uncomplicated and profitable method for removing arsenic and other trace species harmful to health from drinking water, especially drinking water based on low-iron groundwater.